Introduction
In response to tasking by the FAA as defined in a letter dated July 3, 2013, titled, “Request
Formation of Advisory Group to Address Specific Engine and Installation Icing Issues”, the EIWG has
studied the issue of ground operations of turbine engines during heavy snow conditions. This report
provides the short term findings and recommendations. This will be provided to the FAA in response
to their tasking request. It will also be provided to the SAE G12 for consideration when developing
guidance for ground operations in heavy snow. This report addresses short term recommendations
since a more detailed analysis has not yet been accomplished and may take a much longer time
frame to address.
The FAA has participated on this AIA committee; however conclusions stated within this report do
not necessarily represent the views of the FAA. Once this report is submitted to the FAA, the FAA
has stated that they will review the final conclusions, respond to the recommendations and make a
decision as to how to proceed forward.
Aerospace Industries Assoiation
AIA/EIWG Subcommittee on Engine Ground
Operations During Heavy Snow
August 2014
1000 Wilson Boulevard
Suite 1700
Arlington, Va. 22209
www.aia-aerospace.org
2
SECTIONS
1
Conclusions / Recommendations
The task group determined that a list of
recommendations could be developed
and provided to Operators (i.e. airlines) for
consideration during ground operations in
heavy snow. The recommended operating
procedures for ground operations in heavy
snow are discussed in Section 3.3.
2Abbreviations
HOT – Hold Over Time
LWC – Liquid Water Content
LWE – Liquid Water Equivalent
POI – Principal Operations Inspector
TC – Type Certificate
TWC – Total Water Content
3Discussion
The FAA’s Flight Standards Division
regulates aircraft operators’ ground based
operations in icing conditions under 14
CFR 121.629. On an annual basis the
FAA reissues Notice 8900 to provide
guidance to part 121 operators for the
following winter ground icing operations.
The guidance is used by an airline in the
annual update of its ground deicing plan
which it is required to submit to its Flight
Standards principal operations inspector
(POI). The POI must evaluate and
approve the plan. For the winter of
2014-2015, the FAA is considering
incorporating guidance in the notice on
allowing the use of LWE systems to
provide holdover times in heavy snow
operations. The FAA raised the issue of
whether aircraft turbine engines can be
safely operated in these conditions, and if
so, what additional procedures should be
considered.
3
3Discussion: Historic Basis for Engine Snow Requirements
In 2004 and 2005 the FAA sponsored
Aviation Rulemaking Advisory Committee (ARAC)
tasked the Engine Harmonization Working Group
(EHWG) to review propulsion system icing
service experience in order to recommend any
potential changes to the airworthiness
regulations. As a result, the EHWG reviewed and
discussed the basis for snow concentration level
for ground taxi operation certification.
Consequently, they recommended changes
which were proposed to turbine engine icing
airworthiness standards in 14CFR 33.68
(reference FAA Notice of Proposed Rulemaking
10-10) [1]. 14 CFR 33.68 at Amendment 33-10
specifies 0.3 g/m3 LWC for ground taxi operation
in freezing fog, but do not specifically address
snow. Additionally, turbine engine installation
icing airworthiness standards were also reviewed
and revised. The propulsion installation
requirements are contained within 14 CFR
25.1093. Amendment 25-72 of 14 CFR 25.1093
requires operation without adverse effect on
engine operation or serious loss of power or
thrust in “falling and blowing snow within the
limitations established for the airplane for such
operation,” but is not specific about snow
concentration level. This requirement was
originally introduced in amendment 25-36 of part
25.
FAA guidance in advisory circular 20-147 shows
that snow concentration requirements have been
based on visibility correlations [2]. These correlations specify 1/4 statute mile as the boundary
between moderate and heavy snowfall. At the
time, the ¼ mile visibility was assumed to be the
worst case condition for ground operations. The
correlation specifically is:
C = 2100 * V-1.29
where: C = concentration in g/m3
V = visibility in meters
Substituting V = 1/4 mile into the equation
yields a concentration C = 0.91 g/m3.
Figure A, below, is a reproduction of Figure 12
from [3]. It includes two curves indicating
cumulative probability of snow concentrations
calculated from two data sets of observed
visibility observations. Curve A is based on the
same data set used to derive the correlation, and
indicates that the probability is about 94% that
snow concentration will not exceed 0.91 g/m3.
Curve B is based on a larger multiyear data set,
and indicates that the probability is greater than
99% that snow concentration will not exceed
0.91 g/m3.
Figure A.
Reference 1
Data on Snow
Concentration
4
During the EHWG deliberations, Environment
Canada membership offered concern that
visibility has been shown by Rasmussen, et al.
[2] to be a poor indicator of precipitation rate[1].
To check the above numbers, the following
calculation was performed. The maximum
precipitation rate for moderate snow is 2.5 mm/
hr liquid water equivalent. From a Transport
Canada data set of 338,000 minutes of snowfall
data, the 95% and 99% values were 2 and 4
mm/hr, respectively, showing that a 2.5 mm/hr
threshold provides a severe and unlikely
threshold. It was also noted that holdover time
tables for anti-icing fluids are only sanctioned by
the FAA for use in light or moderate snow
conditions. The upper threshold for endurance
time testing for moderate snow is 2.5 mm/hr.
Using a liquid water equivalent rate of 2.5 mm/hr
and a conservative snow flake fall speed
(terminal velocity) of 0.8 m/s, a snow
concentration with an equivalent liquid water
content of 0.9 g/m3 is obtained. The snow fall
speed has a direct effect on the liquid water
equivalent rate for a given snow concentration.
The following table shows the effect for various
fall rates at a constant accumulation rate:
The EHWG deliberated on fall rates and decided
to use a conservative fall rate of 0.8 m/s.
The EHWG further concluded that the two
estimates described above (using visibility
calculations of reference [3] or using a 2.5 mm/hr
accumulation rate along with a 0.8 m/s fall rate)
are similar, and consequently, 0.9 g/m3 became
the recommended level for testing at ground idle
in snow[1]. These considerations underlie the
recommended value of 0.9 g/m3 for Condition
3 in the proposed amendments to 14CFR 33.68
and 14 CFR 25.1093 defined in NPRM 10-10. As
described above, the value of 0.9 g/m3
atmospheric snow concentration was assumed
to correspond to the limit for moderate snow
conditions. Recent developments are challenging
that assumption and ground operations in heavy
snow conditions are recognized as more likely to
occur.
Table 1
Dependence of LWC for Snow on assumed
Fall Speed of Snow
Rate
Fall speed
mm/hr m/s
LWC
g/m3
2.50
2.50
2.50
0.80
1.00
1.50
0.87
0.70
0.46
5.00
5.00
5.00
0.80
1.00
1.50
1.74
1.39
0.93
5
3Discussion: Recent Developments in Ground Operations in Snow
The SAE G12 committee has been working over
the past few years to clarify aircraft anti-icing
hold-over-times (HOT) for operations in snow
[4]. A recent development has occurred where
airline operators may be able to use holdover
times (HOTs) at airports in snow conditions that
exceed moderate snow limits, when they use a
newly developed liquid-water-equivalent (LWE)
system. The LWE system measures LWE rate
and other parameters to determine HOT using
regression curves from endurance time testing
of airframe deicing fluids. The proposed upper
rate for HOT determination by these systems for
snow is 5.0 mm/hr fall rate (equivalent to
50 g/dm2/hr catch rate), which is already
permitted by Transport Canada under an
exemption to their regulations. However, during
the winter of 2013-2014, the FAA has temporarily
established an upper limit for operations in heavy
snowfall as 2.5 mm/hr (equivalent to
25 g/dm2/hr). This temporary limit will be lifted
starting in the winter of 2014-2015. At that time
there will be no upper limit for operation in heavy
snow using the LWES.
Note that the FAA already allows users to
operate in conditions that exceed moderate
snow limits if a visual pre-takeoff aircraft wing
contamination check is performed by the pilot
within 5-minutes of takeoff. This allows
operations in heavy snow conditions, with no
upper limit other than conditions that would
cause an interruption of airport operations.
From table 1 above, it can be seen for the
proposed snow fall accumulation rate of
5.0 mm/hr, the atmospheric concentration is
1.74 g/m3 at a snow fall speed of 0.8 m/s.
For aircraft turbine engine operations in heavy
snow conditions, an atmospheric TWC
concentration could potentially be 1.74 g/m3 or
higher.
It should be noted that aircraft have been
operated in snow conditions for as long as there
have been aircraft engines. Since there has
historically been no generally available, accurate
way of determining snow fall rates and
atmospheric concentrations during operations, it
is likely that aircraft have periodically operated in
heavy snow conditions in the past, with relatively
few safety problems, although safety-significant
events have occurred. Figure 1shows the range
of snowfall rates possible using 1/4 statute mile
visibility as the boundary between moderate
and heavy snowfall. The historical data is from
Rasmussen et al. and the range is defined by the
Fujiyoshi data outline [5]. Figure 1 shows that
snowfall rates of 8 mm/hr may have been
encountered during operations where the
snowfall was considered moderate based on
visibility. This corresponds to an atmospheric
concentration of 2.78 g/m3 based on the
assumptions made in the previous subsection.
The FAA has stated its concern that by utilizing
the relatively new LWE systems, operations in
heavy snow could abruptly increase, resulting in
turbine engines being exposed to levels of snow
above the currently proposed regulations, and
possibly resulting in an increase in
safety-significant engine icing events. This may
be additionally compounded by the fact that
airports or aerodromes are increasingly staying
open during heavier snow conditions, due to
improved snow removal equipment and
procedures. Associated power run-up
procedures to clear engine internal ice accretions
are based on the current freezing ground fog
certification test point of 0.3 g/m3 therefore,
these procedures may not be sufficient to clear
engine ice accretions due to the high
concentrations of snow. Further to this, power
run-up procedures may not clear inlet barrel ice
described in the subsection below.
6
Figure 1:
Range of snowfall rates
possible using 1/4
statute mile visibility
as the boundary
between moderate
and heavy snowfall
3.1 Service Event Data
Appendix 1 contains the list of events collected by the EHWG which are believed to have occurred
as a result of ground operations in heavy snow conditions [1]. In general, the events can be
classified into two categories. The first type of event affects the low-speed spool and symptoms
include fan damage and high vibrations. The second type of events cause core operability issues
and can result in stalls, high vibrations, and in rare instances, compressor damage. It should be
noted that operators are not required to report events to airframe manufacturers, engine
manufacturers, or the aviation authorities unless they surpass thresholds defined by airworthiness
requirements. Thus the database in Appendix 1 may be incomplete.
7
3.1 Service Event Data (Continued)
The low-speed spool events resulting in fan blade damage are believed to be caused by ice
accreting in the inlet barrel of the nacelle, downstream of the ice protection system extent, and
upstream of the fan.29 engines have incurred damage of this nature since 2007 while dispatching
in snow. Fan tip damage events have occurred when visibility was less than 3/4 statute mile, and
strong winds generated blowing snow. Under these conditions, the taxiways can be laden with snow
and melt water that can readily be lifted into the inlet of the engine by the ground vortex, as shown in
Figure 2.After review of pilot statements and photographic evidence, this subcommittee concluded
that taxiway contamination was the root cause of the fan damage. The snow, water and slush was
lifted off the taxiway and deposited on the inlet barrel of the engine where it solidified downstream of
the ice protection system at low engine speeds; this ice was then released when power was
increased. Overall time from push back was determined to be the driving factor for the phenomenon,
and roughly 30 min was required for the threat to materialize for a particular engine and airframe
combination.
Figure 2:
CFM56 ground vortex
generation during testing
at the General Electric
Peebles Test Facility
(The vortex is seen
forming off of the
platform near the lower
intake section of the
engine)
Dispatching during heavy snowfall has also caused core operability issues such as stalls, high
vibrations, and in rare instances, compressor damage. These core damage and operability issues
are believed to be caused by snow impacting and adhering to warm surfaces inside the booster, at
the splitter or IGV to the low pressure compressor. This snow either freezes on these surfaces, or
causes melt water to be generated that can re-freeze downstream. The accretion that is generated
is liberated when the engine power is increased. If the time between run-ups is too long, the
accretion can grow to a size that can lead to core operability events and compressor damage.
8
3.2 Rationale for Recommendations
In the database available to this subcommittee, dispatching in heavy snow has led to the following
engine specific events since 1991: 47fan damage, 7 events were generally classified as engine
damage with one instance specifically related to the core, 4 cases of high vibrations, and 6 stalls.
These events have resulted in rejected take-offs, and air turnarounds. In some cases the damage
was discovered upon inspection at the destination airport. Again, it should be noted this database
may be incomplete as operators are not required to report events unless they surpass thresholds
defined by airworthiness requirements.
Many of the engine events in heavy snow were model specific, or particular to an engine-airframe
combination. However, due to the implementation of the new LWES, and the uncertainty of turbine
engines being exposed to levels of snowfall beyond existing field experience,the recommendations
in the following subsection are made for all turbofan powered transport-category airplane ground
operations.
3.3Recommendations
In addition to the typical best practices listed below, this group recommends inspecting the inlet of
the engines at the central deicing station. Particular emphasis should be placed on the inlet barrel.
If visible from a safe distance, the fan blades, spinner, splitter lip, and inlet guide vanes to the core
should also be inspected. Operating engines should be visually inspection by a qualified Aircraft
Maintenance Engineer from a spotter vehicle. Engines should be operating during the inspection so
that thermal deicing systems remain active, and to maintain facility throughput. The airframe
manufacturer (TC holder) should be consulted for inlet contamination removal procedures.
• Inspect and remove ice from engine inlet and fan area:
• Prior to leaving gate
• Prior to leaving central deicing station
• Cover engine inlet during heavy snow conditions when engines are not operating
• Ensure proper functioning of engine inlet ice protection systems per applicable AFM procedures
• Minimize time period from gate push-back to arrival at deicing station, and from departure from
deicing station to arrival at runway in preparation for take-off roll
• Adjust push-back rates to minimize taxi time during threat conditions
• Taxiways should be kept clear of snow and water
• Deicing stations should be kept clear of snow and water
• Consider having a grating beneath engines to eliminate the ground vortex responsible for
lifting snow, water and diluted deicing fluid into the engine
• Prior to engine start and push-back, verify that engines are free of contamination
• This includes areas of fan blades (typically leading edge and back of blades), engine nacelle
barrel, splitter lip, and core inlet guide vanes
9
3.3 Recommendations (Continued)
•
•
•
•
•
•
• Consult airframe manufacturer (TC holder) for inlet contamination removal procedures
Taxi with all engines running in heavy snow conditions
Perform the AFM recommended procedures for engine power run-ups in icing conditions. Check
with airframe manufacturer (TC holder) to determine if more frequent engine power run-ups would
be prudent in heavy snow ground operations
• It is critical that run up frequency and power sets are adhered to during heavy snow
Avoid engine power increases near puddles, on the wet tarmac or taxi way, or near snow banks
Prior to leaving the central deicing station, verify that engine nacelle barrels are free of
contamination
• Operating engines should be visually inspected by a qualified Aircraft Maintenance Engineer
from a spotter vehicle to maintain facility throughput
• Fan blade and core ice are removed if proper OEM recommended run-up procedures
are adhered to
• Consult airframe manufacturer (TC holder) for inlet contamination removal procedures
Avoid close following of other aircraft during taxi since heavier snow concentrations can enter the
engines
Just prior to take-off roll, follow AFM procedures for pre-takeoff engine power setting
4Acknowledgments
5
Works Cited
The AIA/EIWG Subcommittee on Engine Ground
Operations during Heavy Snow would like to
acknowledge the contributions of the Jason
Brown, John Horrigan (Winter Operations, Air
Canada) and Roy Rasmussen (National Center
for Atmospheric Research).
1. EHWG, “Technical Compendium From Meetings of the Engine Harmonization Working
Group,” DOT/FAA/AR-09/1, Section 3.4
GROUND OPERATION IN SNOW.
2. R. Rasmussen, J. Vivekanandan, J. Cole,
B. Myers and C. Masters, “The Estimation
of Snowfall Rate Using Visibility,” Journal of
Applied Meteorology, vol. 38, pp. 1542-1563,
1999.
3. J. Stallabrass, “Snow Concentration Measurements and Correlation With Visibility,”
in AGARD Conference Proceedings No. 236
Icing Testing for Aircraft Engines, 1978.
4. FAA, “OFFICIAL FAA HOLDOVER TIME TABLES,” Federal Aviation Administration, 2013.
[Online]. Available: https://www.faa.gov/
other_visit/aviation_industry/airline_operators/airline_safety/media/FAA_2013-14_Holdover_Tables.pdf.
5. Y. Fujiyoshi, G. Wakahamam, T. Endoh, S.
Irikawa, H. Konishi and M. Takeuchi, “Simultaneous observation of snowfall intensity and
visibility in winter at Saporro,” Low Temperature Sciences, vol. 42A, p. 147–156, 1983.
10
Appendix 1 Event Data
It should be noted that operators are not required to report events to airframe manufacturers, engine
manufacturers, or the aviation authorities unless they surpass thresholds defined by airworthiness
requirements. Thus the database may be incomplete.
11